My script incorrectly adds 1 extra int to game propert, why?
Your keyboard sensors will keep triggering while the key is pressed, possibly triggering your script twice or more for a single key press. To solve this, enable the "Tap" button so that it only triggers once for a press.

I can also recommend reading up on PEP 8 -- Style Guide for Python Code. It gives you guidance on naming conventions; following it makes it easier to collaborate with others, and also makes it easier for us to understand your code and help you with issues.